The present invention relates generally to an image-forming apparatus and, more specifically, to a coupler that accurately transmits angular motion from the drive gears to a photoconductive drum of an image-forming apparatus.
Image forming devices including copiers, laser printers, facsimile machines, and the like, include a photoconductive drum (hereinafter referred to as drum) having a rigid cylindrical surface that is coated along a defined length of its outer surface. The surface of the drum is charged to a uniform electrical potential and then selectively exposed to light in a pattern corresponding to an original image. Those areas of the photoconductive surface exposed to light are discharged thus forming a latent electrostatic image on the photoconductive surface. A developer material, such as toner, having an electrical charge such that the toner is attracted to the photoconductive surface is brought into contact with the photoconductive surface. A recording sheet, such as a blank sheet of paper, is then brought into contact with the photoconductive surface and the toner thereon is transferred to the recording sheet in the form of the latent electrostatic image. The recording sheet is then heated thereby permanently fusing the toner to it. In preparation for the next image forming cycle, the residual toner is removed from the photoconductive surface.
Combining the drum and toner container into an integral process cartridge mounted in the image forming apparatus is commonly established in the art and provides key advantages. Because toner is consumed in the image forming process, it must be replenished periodically. The drum also wears in the imaging forming process and, with wear, the quality of the formed image deteriorates. Therefore, the drum also must be replaced at defined intervals. Incorporating both the drum and toner supply into a process cartridge allows the end user to replace both toner and drum by removing the old process cartridge from the image forming apparatus and replacing it with a new one.
Precise positioning of the drum with respect to the body of the image forming apparatus is critical for good image quality. Particularly in a color image forming apparatus wherein a plurality of process cartridges are used, positioning between each of the drums with respect to each other, and positioning of all the drums with respect to the body of the image forming apparatus is critical to achieving good image quality.
In many image-forming apparatus, a drive train is permanently mounted and includes a last drive gear that couples to a gear permanently attached to the drum. In one embodiment, the drum gear has a large base diameter several times larger than the diameter of the drum to produce accurate movement since the angular error in the position of the drum would be proportional to displacement errors caused by tooth imperfections and pitch errors and inversely proportional to the base diameter. However, on a cartridge removable by the end user of the machine, the size of the gear permanently mounted to the drum is limited approximately to the drum diameter because otherwise the shape or size of the cartridge would be unwieldy. Therefore, to use a large gear as a final gear in the drum drive train, this gear has to be permanently mounted in the machine, not the removable cartridge. This gear then has to be connected effectively to the removable drum for operation by some kind of coupler.
Because the drum is part of the removable cartridge, the drum may not align with the drive gear when the cartridge is inserted into the image forming apparatus. In particular, the effective axes of rotation of the drum and gear may not be quite parallel to each other, and they may be offset radially by amounts that vary from machine to machine. The coupler should be able to provide accurate rotary motion from the drive gear to the drum despite such misalignments. Also because the drum is removable, the coupler has to disconnect from the drum and reconnect to the drum when the drum is removed and replaced, respectively.
Thus, there exists a need to transmit rotary motion accurately from the drive gear of the image forming apparatus to the drum even when the axes of rotation of the two are misaligned, and this connection has to be broken and reestablished for drum replacement.
The present invention is directed to a coupler for transmitting rotary motion from a drive gear within an image forming apparatus to a driven member, such as a photoconductive drum or an ITM belt drive roller. The coupler provides for accurate transmission of motion, even when the axes of the gear and driven member do not align.
The coupler includes a substantially hollow, elongated body having a first end and a second end. A number of flanges extend from the first end for contacting the drive gear, and prongs extend from the second end for contacting the drum. A biasing member is positioned within the hollow interior section for biasing the coupler from the gear toward the drum.
The invention further includes an image forming apparatus having a main image forming body with a drive gear having a central opening and ridges extending axially along the central opening. A photoconductive drum is mounted within the body and has a substantially cylindrical shape with an end cap on a first end with surfaces to receive drive motion from the coupler. The photoconductive drum is mounted within the body such that the drum end cap is positioned adjacent to the drive gear. A coupler extends between the body drive ridges and the drum end cap and includes a biasing member for maintaining the coupler between the photoconductive drum and the main image forming body.
The drum may include a dome positioned at a middle point of the drum end cap and extending outward for contacting the coupler and maintaining axial location between the drum end cap and body drive spokes. The end of the coupler adjacent the dome may include a contoured edge for aligning the coupler relative to the dome and drum end cap while the drum is inserted into the machine.
Additionally, the same kind of coupler may be used for transmitting rotary motion from a drive gear within an image forming apparatus to an intermediate transfer medium drive roller or gear. The coupler has a similar shape to that previously described and extends between a drive gear within the image forming apparatus and an intermediate transfer medium roller or gear on a removable intermediate transfer medium module.